Hybrid fiber-coaxial (HFC) networks include both fiber optic and coaxial connections and are commonly used to provide broadband data and video services. For example, HFC networks offer broadcast video, interactive television, digital video, high-speed data and Internet access, and telephony services. In a typical implementation, HFC networks deliver such broadband data services as optical signals transmitted from head-end locations via an optical fiber to an optical distribution node. On the downlink side (i.e., head-end location-to-subscriber direction), the optical distribution node converts the optical signals to radio frequency (RF) signals and transmits the RF signals to subscriber locations via coaxial cable links. Some HFC networks allow for two-way communication. Such two-way HFC networks usually include an uplink (i.e., subscriber-to-head-end location direction) communication connection, through which subscriber premises devices transmit RF signals to the optical distribution node that then converts the RF signals to optical signals for transmission back to the head-end location.
The RF signals in an HFC network are typically transmitted in the 5 MHz to 1.8 GHz range. In some implementations, the frequency spectrum from 85 MHz to 1 GHz is used by the optical distribution node for downlink signals, while the frequency spectrum from 5 to 85 MHz is used by the subscriber premises devices for uplink signals. However, the actual split of the spectral band can vary by the standard used. Using such configurations, an HFC network can provide adequate two-way services for interactive services, such as Internet access, e-mail, voice/telephone services, or video on demand. However, the two-way services provided by existing HFC networks are often limited by the bandwidth asymmetry in the downlink and uplink frequency spectra. As new two-way applications and services become more popular, the demand for HFC networks to provide symmetric high-quality high-bandwidth services also increases. Interactive video, interactive gaming, video telephony, videoconferencing, remote storage, virtual DVD, and high-speed virtual private networks (VPNs) are just a few such applications for existing two-way HFC networks may be inadequate. Increases in uplink, or so-called “return path,” traffic have exposed limitations in the current HFC networks. In some implementations, the optical link devices required to convert the RF uplink signals to optical signals are the limiting factor.
In many HFC networks, the return path signal is converted from the electrical domain to the optical domain in the optical distribution node using a directly modulated semiconductor laser, such as diode laser. The simplicity of such implementations comes at the cost of very stringent noise and distortion requirements on the laser. More complex, and consequently more expensive, distributed feedback (DFB) lasers have been proven to meet the voice and distortion requirements. While the additional cost associated with DFB lasers can sometimes be justified in implementations in which extremely large populations of end users are served, such lasers still have limitations. While the channel capacity of DFB lasers is large, such devices are still inherently limited by the physical characteristics of the semiconductor laser. If the product of the number of channels and modulation depth per channel exceeds the threshold current of the laser, the modulation current will drop below the laser threshold current and shut off the laser. For example, if the RF input signal power to the laser goes beyond an inherent or characteristic limit, then signal distortion increases rapidly. Also, if RF input signal power goes substantially above that limit, distortion known as “laser clipping” occurs.
Laser clipping occurs when the modulating RF input signal current that drives the semiconductor laser, in either the uplink or downlink directions, occasionally falls below the laser's threshold current resulting in nearly zero optical power output. This behavior degrades the Bit Error Rate (BER) performance of the transmitted digital signal.